Epithelium is a sheet of tightly packed cells that covers every surface facing the outside world or a hollow space. Its shape and number of layers match its job: protection, absorption, secretion or exchange.
A sheet on a base
Epithelial cells sit on a thin basement membrane and are joined tightly to each other. They have no blood vessels of their own and get nutrients by diffusion from the tissue below.
Shapes and layers
Cells are flat (squamous), cube-shaped (cuboidal) or tall (columnar). One layer is called simple and suits exchange or absorption; many layers are called stratified and suit areas that take wear.
Special linings
The airways have a ciliated lining that sweeps mucus upward. The bladder has a stretchy lining that lets it fill and empty. Epithelium renews quickly, so it heals fast after small injuries.
Glands
Glands are epithelium that makes and releases substances. Exocrine glands release through ducts, like sweat and salivary glands; endocrine glands release hormones into the blood.
| Type | Example site | Main job |
|---|---|---|
| Simple squamous | Alveoli, capillary walls | Fast exchange of gases and fluid |
| Simple columnar | Stomach and intestine | Absorption and secretion |
| Stratified squamous | Skin, mouth, esophagus | Protection from wear |
| Pseudostratified ciliated | Trachea and bronchi | Moving mucus out of the airway |
At the bedside
Swabs, scrapings and many biopsies sample epithelium, so a good sample needs firm contact with the lining. Most cancers in adults, called carcinomas, start in epithelial cells.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).